Since many years, SCHOTT has continuously published a broad range of statistical data on bending strength of ZERODUR<sup>®</sup>. With this data it is possible to do lifetime calculations utilizing on threshold values for the bending strength of ZERODUR<sup>®</sup> under various given surface treatments and environmental conditions. The bending strength of ZERODUR<sup>®</sup> is mainly a function of the sub-surface damage (SSD) residues after final surface processing. Due to the nature of the bending strength of brittle materials, a surface that is free of sub-surface damage will have a significantly higher strength, compared to a surface with remaining SSD. Glass surfaces free of sub-surface damage need to be actively protected or handled with care to avoid any new defects like scratches that subsequently would reduce the strength of the surface. However, there is no data available that answers the questions: “Is every visible scratch automatically reducing the strength of the surface?”, “How easy is it to generate scratches on a ZERODUR<sup>®</sup> surface?”. This publication discusses various sub-surface damage measurements on ZERODUR<sup>®</sup> surfaces in relation to their surface treatment, including artificially generated scratches on polished and acid etched surfaces.

The coefficient of thermal expansion of ZERODUR<sup>®</sup> is optimized for an application temperature between 0&deg;C to 50&deg;C. Nevertheless ZERODUR<sup>®</sup> can also be used at other temperatures. It has been shown elsewhere, that ZERODUR<sup>®</sup> can be used at deep, down to cryogenic temperatures. At higher temperatures, the use of ZERODUR<sup>®</sup> is restricted by the required accuracy of the coefficient of thermal expansion. As given in the ZERODUR<sup>®</sup> catalog, the maximum application temperature is 600&deg;C. Slight changes of the CTE may occur if ZERODUR<sup>®</sup> is cooled down from application temperatures between 130&deg;C and 320&deg;C with rates that differ from the initial production annealing rate. This can be relevant in coating processes that work at temperatures above 130&deg;C. In this paper, the effects of high temperature treatments on ZERODUR<sup>®</sup> properties are discussed. Advices are given how to avoid material property or geometric changes if heat treatment cannot be avoided.

More and more applications utilize the short wave infrared (SWIR) spectral range. The SWIR range is defined from about 0.9 to 3 &mu;m. SWIR applications can be found for example in inspection processes of circuit boards, solar cells, bottles and food. The SWIR range is used in identification, sorting, surveillance, inspection and more. With SWIR applications characteristics can be visualized that normally would not be detectable with visible light, like rotten fruits in fruit sorting, fakes in paintings, content levels in visually non-transmitting bottles. For all these machine vision applications specific optics are used that in the ideal case have transmittance in the visual spectral range and in the SWIR range. Optical designs require materials that are transmittance in the visible and the SWIR range, sometimes even up to 4 &mu;m. Most optical glasses have a good transmittance up to 2 &mu;m, but at 2.5 &mu;m the transmittance strongly decreases. There is also not much available information on the details of the transmittance curve in the SWIR range available of optical glass. This presentation will demonstrate SCHOTT optical glasses with good transmittance even up to 4 &mu;m. If applications require transmittance at even larger wavelengths, it is possible to utilize IRG infrared material with transmittance up to 8 µm, that still can be used in the visible down to 0.6 &mu;m. Other critical information for the optical designs are the dispersion and index change with temperature (dn/dT) characteristics in the SWIR range of these materials. This paper will discuss the availability of such data.

Optical systems in space environment have to withstand harsh radiation. Radiation in space usually comes from three main sources: the Van Allen radiation belts (mainly electrons and protons); solar proton events and solar energetic particles (heavier ions); and galactic cosmic rays (gamma- or x-rays). Other heavy environmental effects include short wavelength radiation (UV) and extreme temperatures (cold and hot). Radiation can damage optical glasses and effect their optical properties. The most common effect is solarization, the decrease in transmittance by radiation. This effect can be observed for UV radiation and for gamma or electron radiation. Optical glasses can be stabilized against many radiation effects. SCHOTT offers radiation resistant glasses that do not show solarization effects for gamma or electron radiation. A review of SCHOTT optical glasses in space missions shows, that not only radiation resistant glasses are used in the optical designs, but also standard optical glasses. This publication finishes with a selection of space missions using SCHOTT optical glass over the last decades.

The coefficient of thermal expansion (CTE) and its spatial homogeneity from small to large formats is the most important property of ZERODUR. Since more than a decade SCHOTT has documented the excellent CTE homogeneity. It started with reviews of past astronomical telescope projects like the VLT, Keck and GTC mirror blanks and continued with dedicated evaluations of the production. In recent years, extensive CTE measurements on samples cut from randomly selected single ZERODUR parts in meter size and formats of arbitrary shape, large production boules and even 4 m sized blanks have demonstrated the excellent CTE homogeneity in production. The published homogeneity data shows single ppb/K peak to valley CTE variations on medium spatial scale of several cm down to small spatial scale of only a few mm mostly at the limit of the measurement reproducibility. This review paper summarizes the results also in respect to the increased CTE measurement accuracy over the last decade of ZERODUR production.

The fluorescence of optical glasses is a property that needs to be taken into account in optical designs for life science applications. Many optical glasses from SCHOTT show a very low intrinsic or auto-fluorescence. The fluorescence depends mainly on the applied excitation wavelength and the optical glass type. The fluorescence of optical glasses is usually defined as the quotient of the integral of the emission spectrum with the integral of the emission spectrum of a reference glass. This definition does not give any information about the actual quantum efficiency of the fluorescence. In this presentation recent data on the integral fluorescence of SCHOTT optical glasses are presented. Additionally, first measurements of the quantum efficiency of SCHOTT optical glasses are presented and compared to the standard method.

Femtosecond lasers are more and more used for material processing and lithography. Femtosecond laser help to generate three dimensional structures in photoresists without using masks in micro lithography. This technology is of growing importance for the field of backend lithography or advanced packaging. Optical glasses used for beam shaping and inspection tools need to withstand high laser pulse energies. <p> </p>Femtosecond laser radiation in the near UV wavelength range generates solarization effects in optical glasses. In this paper results are shown of femtosecond laser solarization experiments on a broad range of optical glasses from SCHOTT. The measurements have been performed by the Laser Zentrum Hannover in Germany. The results and their impact are discussed in comparison to traditional HOK-4 and UVA-B solarization measurements of the same materials. The target is to provide material selection guidance to the optical designer of beam shaping lens systems.

SCHOTT has introduced IRG 27, a classic chalcogenide glass (As<sub>2</sub>S<sub>3</sub>), possessing transparency from approximately 0.65 μm to well into the long-wavelength IR (&gt; 10 μm). Additionally, this glass exhibits an extremely small thermo-optic (dn/dT) value for most of its transparency range, a critical attribute in certain applications. Relatively low density and a good thermal expansion match to aluminum are further useful characteristics of IRG 27.

Large amounts of low thermal expansion material are required for the upcoming ELT projects. The main mirror is designed using several hundreds of hexagonal 1.4 m sized mirror blanks. The M2 and M3 are monolithic 4 m class mirror blanks. The mirror blank material needs to fulfill tight requirements regarding CTE specification and homogeneity. <p> </p>Additionally the mirror blanks need to be dimensionally stable for more than 30 years. In particular, stress effects due to the changes in the environment shall not entail shape variation of more than 0.5 &mu;m PV within 30 years. <p> </p>In 2010 SCHOTT developed a physically based model to describe the thermal and mechanical long time behavior of ZERODUR. The model enables simulation of the long time behavior of ZERODUR mirror blanks under realistic mechanical and thermal constraints. This presentation shows FEM simulation results on the long time behavior of the ELT M1, M2 and M3 mirror blanks under different loading conditions. Additionally the model results will be compared to an already 15 years lasting long time measurement of a ZERODUR sample at the German federal physical standardization institute (PTB).<p> </p> In recent years SCHOTT pushed the push rod dilatometer measurement technology to its limit. With the new Advanced Dilatometer CTE measurement accuracies of +- 3 ppb/K and reproducibilities of better 1 ppb/K have been achieved. The new Advanced Dilatometer exhibits excellent long time stability.

The new ground based telescope generation is moving to a next stage of performance and resolution. Mirror substrate material properties tolerance and homogeneity are getting into focus. The coefficient of thermal expansion (CTE) homogeneity is even more important than the absolute CTE. The error in shape of a mirror, even one of ZERODUR, is affected by changes in temperature, and by gradients in temperature. Front to back gradients will change the radius of curvature R that in turn will change the focus. Some systems rely on passive athermalization and do not have means to focus. Similarly changes in soak temperature will result in surface changes to the extent there is a non-zero coefficient of thermal expansion. When there are in-homogeneities in CTE, the mirror will react accordingly. Results of numerical experiments are presented discussing the impact of CTE in-homogeneities on the optical performance of 4 m class mirror substrates. Latest improvements in 4 m class ZERODUR CTE homogeneity and the thermal expansion metrology are presented as well.

The upcoming extremely large telescope projects like the E-ELT, TMT or GMT telescopes require not only large amount of mirror blank substrates but have also sophisticated instrument setups. Common instrument components are atmospheric dispersion correctors that compensate for the varying atmospheric path length depending on the telescope inclination angle. These elements consist usually of optical glass blanks that have to be large due to the increased size of the focal beam of the extremely large telescopes. <p> </p>SCHOTT has a long experience in producing and delivering large optical glass blanks for astronomical applications up to 1 m and in homogeneity grades up to H3 quality in the past. <p> </p>The most common optical glass available in large formats is SCHOTT N-BK7. But other glass types like F2 or LLF1 can also be produced in formats up to 1 m. The extremely large telescope projects partly demand atmospheric dispersion components even in sizes beyond 1m up to a range of 1.5 m diameter. The production of such large homogeneous optical glass banks requires tight control of all process steps. <p> </p>To cover this demand in the future SCHOTT initiated a research project to improve the large optical blank production process steps from melting to annealing and measurement. Large optical glass blanks are measured in several sub-apertures that cover the total clear aperture of the application. With SCHOTT's new stitching software it is now possible to combine individual sub-aperture measurements to a total homogeneity map of the blank. In this presentation first results will be demonstrated.

The Semiconductor Industry is making continuous progress in shrinking feature size developing technologies and process to achieve &lt; 10 nm feature size. The required Overlay specification for successful production is in the range one nanometer or even smaller. Consequently, materials designed into metrology systems of exposure or inspection tools need to fulfill ever tighter specification on the coefficient of thermal expansion (CTE). The glass ceramic ZERODUR® is a well-established material in critical components of microlithography wafer stepper and offered with an extremely low coefficient of thermal expansion, the tightest tolerance available on market. SCHOTT is continuously improving manufacturing processes and it’s method to measure and characterize the CTE behavior of ZERODUR®. This paper is focusing on the "Advanced Dilatometer" for determination of the CTE developed at SCHOTT in the recent years and introduced into production in Q1 2015. The achievement for improving the absolute CTE measurement accuracy and the reproducibility are described in detail. Those achievements are compared to the CTE measurement accuracy reported by the Physikalische Technische Bundesanstalt (PTB), the National Metrology Institute of Germany. The CTE homogeneity is of highest importance to achieve nanometer precision on larger scales. Additionally, the paper presents data on the short scale CTE homogeneity and its improvement in the last two years. The data presented in this paper will explain the capability of ZERODUR® to enable the extreme precision required for future generation of lithography equipment and processes.

Extremely low dispersion glasses (e.g. SCHOTT XLD glasses) play an essential role in the color correction of optical systems. Together with short flint glasses (KZFS Types) they can be used for apochromatic color correction in the visible spectrum or even for broadband color correction in combination with lanthanum crown glasses (LAK Types). <p> </p>Unfortunately the chemical composition of those glasses leads to a high coefficient of thermal expansion, low hardness and low resistance against chemical attacks. As a consequence these glasses tend to be difficult in processing. Therefore the glass engineer’s task is to improve processing characteristics while keeping their special optical properties. N-FK58 XLD is an example of a new generation of XLD glasses from SCHOTT with improved workability. In 2014 a processing study has been conducted to optimize the polishing of XLD glasses. This presentation will show the results of this study for N-FK58 XLD and the application to other fluorophosphates glasses.

Highly chromatic corrected optical systems rely on optical glasses with precise optical positions represented by refractive index and Abbe number. A modern production of optical glasses requires an economical, fast and accurate way of monitoring its fabrication. We demonstrate that an automated Hilger-Chance type refractometer fulfills all these needs. Therefore the uncertainty of a set of optical glasses is analyzed on the basis of a high number and long time reproducibility measurements. It turns out that the standard deviations after several hundreds of measurements taken over almost an decade in refraction is better than 10<sup>-5</sup> in refraction and 0.02% in dispersion.

In the recent years, the ever tighter tolerance for the Coefficient of thermal expansion (CTE) of IC Lithography component materials is requesting significant progress in the metrology accuracy to determine this property as requested. ZERODUR® is known for its extremely low CTE between 0°C to 50°C. The current measurement of the thermal expansion coefficient is done using push rod dilatometer measurement systems developed at SCHOTT. In recent years measurements have been published showing the excellent CTE homogeneity of ZERODUR® in the one-digit ppb/K range using these systems. The verifiable homogeneity was limited by the CTE(0°C, 50°C) measurement repeatability in the range of ± 1.2 ppb/K of the current improved push rod dilatometer setup using an optical interferometer as detector instead of an inductive coil. With ZERODUR® TAILORED, SCHOTT introduced a low thermal expansion material grade that can be adapted to individual customer application temperature profiles. The basis for this product is a model that has been developed in 2010 for better understanding of the thermal expansion behavior under given temperature versus time conditions. The CTE behavior predicted by the model has proven to be in very good alignment with the data determined in the thermal expansions measurements. The measurements to determine the data feeding the model require a dilatometer setup with excellent stability and accuracy for long measurement times of several days. In the past few years SCHOTT spent a lot of effort to drive a dilatometer measurement technology based on the push rod setup to its limit, to fulfill the continuously demand for higher CTE accuracy and deeper material knowledge of ZERODUR®. This paper reports on the status of the dilatometer technology development at SCHOTT.

ZERODUR<sup>® </sup>glass ceramic from SCHOTT is known for its very low thermal expansion coefficient (CTE) at room
temperature and its excellent CTE homogeneity. It is widely used for ground-based astronomical mirrors but also for
satellite applications. Many reference application demonstrate the excellent and long lasting performance of
ZERODUR<sup>®</sup> components in orbit. For space application a low CTE of the mirror material is required at cryogenic
temperatures together with a good match of the thermal expansion to the supporting structure material.
It is possible to optimize the coefficient of thermal expansion of ZERODUR<sup>®</sup> for cryogenic applications. This paper
reports on measurements of thermal expansion of ZERODUR<sup>®</sup> down to cryogenic temperatures of 10 K performed by
the PTB (Physikalisch Technische Bundesanstallt, Braunschweig, Germany, the national metrology laboratory). The
ZERODUR<sup>®</sup> TAILORED CRYO presented in this paper has a very low coefficient of thermal expansion down to 70 K.
The maximum absolute integrated thermal expansion down to 10 K is only about 20 ppm.
Mirror blanks made from ZERODUR<sup>® </sup>TAILORED CRYO can be light weighted to almost 90% with our modern
processing technologies. With ZERODUR<sup>®</sup> TAILORED CRYO, SCHOTT offers the mirror blank material for the next
generation of space telescope applications.

The tip and tilt M5 mirror of the European Extremly Large Telescope (E-ELT) requires a demanding approach in light
weighting. The approximately 3 m x 2.5 m elliptical plano mirror is specified to a weight of less than 500 kg with high
Eigenfrequencies and low deformation under different inclination angles.
In 2011 SCHOTT has presented a study to develop a design for the M5 mirror blank of the ESO E-ELT. The design
presented was based on a radial square design to achieve the best compromise between performance and
manufacturability. With the fabrication of a prototype section SCHOTT demonstrated its capability to manufacture the
demanding features including pockets with 350 mm depth, thin walls and sloped pocket bottoms.
Now 3 years later SCHOTT presents an iso-grid based design that is in accordance with the manufacturability progress
that has been demonstrated in various ELZM (Extremely Lightweighted ZERODUR Mirrors) publications in the last two
years. The achievements on the specified mechanical parameters are compared to the first approach from 2011. In this
paper the results are presented and the performance parameters are discussed.

The Daniel K. Inouye Solar Telescope (DKIST, formerly the Advanced Technology Solar Telescope, ATST)
will be the most powerful solar telescope in the world. It is currently being built by the Association of
Universities for Research in Astronomy (AURA) in a height of 3000 m above sea level on the mountain
Haleakala of Maui, Hawaii. The primary mirror blank of diameter 4.26 m is made of the extremely low thermal
expansion glass ceramic ZERODUR® of SCHOTT AG Advanced Optics.
The DKIST primary mirror design is extremely challenging. With a mirror thickness of only 78 to 85 mm it is
the smallest thickness ever machined on a mirror of 4.26 m in diameter. Additionally the glassy ZERODUR®
casting is one of the largest in size ever produced for a 4 m class ZERODUR® mirror blank. The off axis
aspherical mirror surface required sophisticated grinding procedures to achieve the specified geometrical
tolerance. The small thickness of about 80 mm required special measures during processing, lifting and
transport. Additionally acid etch treatment was applied to the convex back-surface and the conical shaped outer
diameter surface to improve the strength of the blank. This paper reports on the challenging tasks and the
achievements on the material property and dimensional specification parameter during the production of the
4.26 m ZERODUR® primary mirror blank for AURA.

The glass ceramic ZERODUR® from SCHOTT has an excellent reputation as mirror blank material for
earthbound and space telescope applications. It is known for its extremely low coefficient of thermal expansion
(CTE) at room temperature and its excellent CTE homogeneity. Recent improvements in CNC machining at
SCHOTT allow achieving extremely light weighted substrates up to 90% incorporating very thin ribs and face
sheets. In 2012 new ZERODUR® grades EXPANSION CLASS 0 SPECIAL and EXTREME have been released
that offer the tightest CTE grades ever. With ZERODUR® TAILORED it is even possible to offer ZERODUR®
optimized for customer application temperature profiles. In 2013 SCHOTT started the development of a new
dilatometer setup with the target to drive the industrial standard of high accuracy thermal expansion metrology
to its limit. In recent years SCHOTT published several paper on improved bending strength of ZERODUR® and
lifetime evaluation based on threshold values derived from 3 parameter Weibull distribution fitted to a multitude
of stress data. ZERODUR® has been and is still being successfully used as mirror substrates for a large number
of space missions. ZERODUR® was used for the secondary mirror in HST and for the Wolter mirrors in
CHANDRA without any reported degradation of the optical image quality during the lifetime of the missions.
Some years ago early studies on the compaction effects of electron radiation on ZERODUR® were re analyzed.
Using a more relevant physical model based on a simplified bimetallic equation the expected deformation of
samples exposed in laboratory and space could be predicted in a much more accurate way. The relevant
ingredients for light weighted mirror substrates are discussed in this paper: substrate material with excellent
homogeneity in its properties, sufficient bending strengths, space radiation hardness and CNC machining
capabilities.

The IC Lithography roadmap foresees manufacturing of devices with critical dimension of &lt; 20 nm. Overlay
specification of single digit nanometer asking for nanometer positioning accuracy requiring sub nanometer
position measurement accuracy. The glass ceramic ZERODUR<sup>®</sup> is a well-established material in critical
components of microlithography wafer stepper and offered with an extremely low coefficient of thermal
expansion (CTE), the tightest tolerance available on market. SCHOTT is continuously improving manufacturing
processes and it’s method to measure and characterize the CTE behavior of ZERODUR<sup>®</sup> to full fill the ever
tighter CTE specification for wafer stepper components. In this paper we present the ZERODUR<sup>® </sup>Lithography
Roadmap on the CTE metrology and tolerance. Additionally, simulation calculations based on a physical model
are presented predicting the long term CTE behavior of ZERODUR<sup>®</sup> components to optimize dimensional
stability of precision positioning devices. CTE data of several low thermal expansion materials are compared
regarding their temperature dependence between - 50&deg;C and + 100&deg;C. ZERODUR<sup>®</sup> TAILORED 22&deg;C is full
filling the tight CTE tolerance of +/- 10 ppb / K within the broadest temperature interval compared to all other
materials of this investigation. The data presented in this paper explicitly demonstrates the capability of
ZERODUR<sup>®</sup> to enable the nanometer precision required for future generation of lithography equipment and
processes.

In high end optical designs the quality of the optical system not only depends on the chosen optical glasses but also on
the available refractive index and Abbe number tolerances. The primary optical design is based on datasheet values of
the refractive index and Abbe number. In general the optical position of the delivered glass will deviate from the catalog
values by given tolerances due to production tolerances. Therefore in many cases the final optical design needs to be
modified based on real glass data. Tighter refractive index and Abbe number tolerances can greatly reduce this additional
amount of work.
The refractive index and Abbe number of an optical glass is a function of the chemical composition and the annealing
process. Tight refractive index tolerances require not only a close control and high reliability of the melting and fine
annealing process but also best possible material data. These data rely on high accuracy measurement and accurate
control during mass production. Modern melting and annealing procedure do not only enable tight index tolerances but
also a high homogeneity of the optical properties.
Recently SCHOTT was able to introduce the tightest available refractive index and Abbe number tolerance available in
the market: step 0.5 meaning a refractive index tolerance of +/- 0.0001 and an Abbe number tolerance of +/- 0.1%. This
presentation describes how the refractive index depends on the glass composition and annealing process and describes
the requirements to get to this tightest refractive index and Abbe number tolerance.

In 2010, SCHOTT introduced a method for the modeling of the thermal expansion behavior of ZERODUR<sup>®</sup> under arbitrary
temperature profiles for an optimized production of material for the upcoming Extremely Large Telescope (ELT) projects.
In 2012 a new product was introduced based on this method called ZERODUR<sup>®</sup> TAILORED. ZERODUR<sup>®</sup> TAILORED
provides an evolution in the specification of the absolute Coefficient of Thermal Expansion (CTE) value by including the
individual customer requirements in this process. This paper presents examples showing the benefit of an application
oriented approach in the design of specifications using ZERODUR<sup>®</sup>. Additionally it will be shown how the modeling
approach has advanced during the last years to improve the prediction accuracy on long time scales.
ZERODUR<sup>®</sup> is known not only for its lowest CTE but also for its excellent CTE homogeneity as shown in the past for disc
shaped blanks typical for telescope mirror substrates. Additionally this paper presents recent results of CTE homogeneity
measurements in the single digit ppb/K range for a rectangular cast plate proving that the excellent CTE homogeneity is
independent of the production format.

The zero expansion glass ceramic ZERODUR<sup>&reg;</sup> is a well-established material in microlithography in critical components as wafer- and reticle-stages, mirrors and frames in the stepper positioning and alignment system. The very low coefficient of thermal expansion (CTE) and its extremely high CTE homogeneity are key properties to achieve the tight overlay requirements of advanced lithography processes. SCHOTT is continuously improving critical material properties of ZERODUR<sup>&reg;</sup> essential for microlithography applications according to a roadmap driven by the ever tighter material specifications broken down from the customer roadmaps. This paper will present the SCHOTT Roadmap for ZERODUR<sup>&reg;</sup> material property development. In the recent years SCHOTT established a physical model based on structural relaxation to describe the coefficient of thermal expansion’s temperature dependence. The model is successfully applied for the new expansion grade ZERODUR<sup>&reg;</sup> TAILORED introduced to the market in 2012. ZERODUR<sup>&reg;</sup> TAILORED delivers the lowest thermal expansion of ZERODUR<sup>&reg;</sup> products at microlithography tool application temperature allowing for higher thermal stability for tighter overlay control in IC production. Data will be reported demonstrating the unique CTE homogeneity of ZERODUR<sup>&reg;</sup> and its very high reproducibility, a necessary precondition for serial production for microlithography equipment components. New data on the bending strength of ZERODUR<sup>&reg;</sup> proves its capability to withstand much higher mechanical loads than previously reported. Utilizing a three parameter Weibull distribution it is possible to derive minimum strength values for a given ZERODUR<sup>&reg;</sup> surface treatment. Consequently the statistical uncertainties of the earlier approach based on a two parameter Weibull distribution have been eliminated. Mechanical fatigue due to stress corrosion was included in a straightforward way. The derived formulae allows calculating life time of ZERODUR<sup>&reg;</sup> components for a given stress load or the allowable maximum stress for a minimum required life time.

Modern pulsed laser applications cover a broad range of wavelength, power and pulse widths. Beam guiding optics in laser systems do not only have specific requirements on the imaging quality but also have to withstand high laser power. The laser damage threshold of an optical component depends on the surface (polishing, coating ...) and also on the bulk material properties. Actual values of bulk laser damage thresholds, particularly at pulse lengths less than 1 nanosecond (1 ns), of optical glasses are rarely found in literature, except for fused silica, which is known as a key optical material for components in high power laser. However, fused silica is rather expensive and limited in optical properties. That is the reason why customers often ask for laser damage threshold data of optical glasses. Therefore, SCHOTT has started a project for the characterization of the bulk laser damage threshold of optical glasses at the wavelengths 532 nm and 1064 nm with pulse lengths in the nano- and pico-second range. Bulk and surface laser damage testing has been performed by the Laser Zentrum Hannover in Germany according to the S-on-1 test of DIN EN ISO 11254-2 / DIN EN ISO 21254.

The two Extremely Large Telescopes under discussion, the Thirty Meter Telescope and the European Extremely Large Telescope, will use a multitude of hexagonal shaped mirror segments to achieve the large aperture of 30 m and 39 m, respectively. The proper functionality of both telescopes will strongly depend upon the variation of material properties between individual segments.
SCHOTT has a well proven experience in production of mirror substrates for segmented telescopes. Today five of the world's six segmented telescopes are using ZERODUR® as mirror substrate material. Since 2003 SCHOTT delivered more than 260 mirrors of 1.5 m in diameter for industrial application not related to astronomy. In this paper the achievements during the serial production of those are presented for specified material properties and dimensional parameters. The presentation includes data on the fulfillment of the CTE specification, the achieved tolerances on surface figure and flatness and other geometrical quality parameters. The data to be presented will demonstrate the excellent reproducibility of ZERODUR®'s material properties and its manufacturing process. The production capabilities at SCHOTT for the successful delivery in time of the multitude of ZERODUR® segments are presented and discussed. They will demonstrate that ZERODUR® is well prepared for the demands of industrial scale production for the two large segmented ELT's in quality, quantity, and in the requested time period.

Previously we established that the combination of new machining parameters for openbacked
monolithic lightweighted ZERODUR® mirror substrates, coupled with new methods for optical
finishing of aggressively lightweighted mirrors, have relieved classical mirror design constraints imposed
upon 1.2m diameter lightweight mirrors. We demonstrate that openback mirror substrates now offer
comparable mass, Eigenfrequency and substructure print-through performance to the sandwich-mirror
architecture, but with considerably less manufacturing effort than for sandwich mirrors. Here we extend
the analyses of the first paper from 1.2m diameter down to 0.6m in diameter and up to 2.4m diameter and
4m diameter mirrors.

In the preceding part I of this paper stressed mirror polishing was stated as one of the processes assumed for the polishing of
non-axisymmetric mirror blanks like those for the two ELT projects (the ESO E-ELT and the TMT). For this process it is
important to have a precise knowledge of the elastic behavior of the glass ceramic mirror substrate materials. In reality
glasses and glass ceramics do not react instantaneously to stresses at room temperature. This effect is called "delayed
elasticity".
It was shown that the delayed elasticity effect of ZERODUR<sup>&#174;</sup> is small in size (less than approximately 1% of the applied
deformation) and fully reversible in time. A mathematical model on the relaxation of shear modulus and bulk modulus of
ZERODUR<sup>&#174;</sup> has been introduced to predict the delayed elasticity at room temperature and different load cases. This second
paper is focusing on an updated model approach with the target to improve the model prediction accuracy. The model
results will be compared to measurements of the effect on a 1.5 m E-ELT mirror blank at L-3 Communications, Tinsley.

ZERODUR<sup>®</sup> is a well-established material in astronomy and all fields of applications where temperature gradients
might limit extreme precision and stability. Together with its rich heritage come a series of recent developments, which
reveal the potential of the material for broader and more demanding applications. The outstanding degree of light-weighting
achieved with progress in CNC grinding in the last two years shows its high suitability for space telescope
mirrors. This is supported by new data on strength enabling higher mechanical loads.
Also ground based telescopes benefit from the improved light-weight processing such as solar telescopes and
downstream mirrors of extremely large telescopes. More and better data have been collected demonstrating the unique
CTE homogeneity of ZERODUR<sup>®</sup> and its very high reproducibility a necessary precondition for large series mirror
production. Deliveries of more than 250 ZERODUR mirrors of 1.5 m in diameter prove the availability of robust
industrial serial production capability inevitable for ELT mirror segment production.

Recent game-changing technology greatly extends the design possibilities and range of applications for aggressively lightweighted open-back Zerodur&reg; mirrors. We have compared several lightweighting design approaches under this new technology. Analytic comparisons are for 1.2m mirrors, all constrained to have a free-free first Eigenfrequency of 200 Hz. Figures of merit include resulting mass, thickness and relative cost. Much more aggressive masses are now available in open-back mirrors, competitive with the more expensive closed-back sandwich mirrors. These breakthroughs are relevant to spaceborne implementation of lightweight mirrors ranging from a few tenths of a meter in diameter to up to 4 meters in diameter.

The first monolithic ZERODUR&reg; 4 m class mirror was ordered by the German Max Planck Institute for Astronomical
Physics in 1968. A ratio of 1:6 for thickness to diameter ratio ensured the necessary stiffness to minimize deformation
under gravity load. The technological ability to actively compensate the bending of the mirror substrate under gravity
initiated the development from heavy non active thick mirror substrates to ever thinner thicknesses starting with the
NTT, the New Technology Telescope of ESO.
The thinner the mirror substrates are becoming the more demanding are the requests on homogeneity of material
properties to ensure best performance over the clear aperture at every spot.
In this paper we present results on material properties achieved for the 4 m class mirror substrates recently delivered by
SCHOTT. The CTE homogeneity, the internal quality regarding striae, bubbles and inclusions as well as stress
birefringence data are reported. Improvements in CNC processing and overall manufacturing process for the very thin 4
m class blanks are discussed.

In the recent past, SCHOTT has demonstrated its ability to manufacture large light weighted ZERODUR&reg; mirror blanks
for telescope projects like the GREGOR solar-telescope, for example. In 2010, SCHOTT was commissioned with a
study aimed at developing a design for the M5 mirror blank of the ESO E-ELT.
The tip and tilt M5 mirror of the European Extremely Large Telescope (E-ELT) requires a demanding approach in light
weighting. The approximately 3.1 m x 2.5 m elliptical plano mirror is specified to a weight of less than 500 kg with high
Eigenfrequencies and low deformation under different inclination angles.
The study was divided into two parts. The first part focused on coming up with an optimized light weighted design with
respect to performance and processability with finite element modeling. In the second part of the study, a concept for the
processing sequence including melting, cold-processing, acid etching and handling of the M5 blank was developed. By
producing a prototype section, SCHOTT demonstrated its ability to manufacture the demanding features, including
pockets 350 mm in depth, thin walls and sloped pocket bottoms.
This paper outlines the results of the design work, processing concept and demonstrator fabrication.

Stress mirror polishing is considered as one of several polishing technologies for the generation of the aspherical shaped
primary mirror segments of the thirty meter telescope (TMT). For stress mirror polishing it is essential to precisely
know the elastic response of glass ceramic substrate materials under a given deformation load. In the past it was
experimentally shown that glass ceramics do not respond instantaneously to loading and unloading conditions, this
effect was called "delayed elasticity."
Recently SCHOTT has shown that it is possible to use a model to predict the characteristic thermal expansion behaviour
of individual ZERODUR&reg; batches for a given temperature profile. A similar approach will be used to predict the
delayed elastic behavior of ZERODUR&reg; under time dependent loads.
In this presentation the delayed elasticity effect of ZERODUR&reg; is reviewed. The delayed elastic response of the
material to load conditions is shown and discussed. First results of a model approach based on experimental results and
tools that have been built up for the modelling of the delayed elasticity effect of ZERODUR&reg; will be presented.

In 2010 ESO will celebrate the 10th anniversary of the fourth 8 m telescope Yepun's first light event. Together with the
other VLT telescopes it has accumulated more than 40 years of extremely successful operation time for astronomy.
Progress in rocket technology and in ZERODUR&reg; light weighting gives reason for contemplating about the use of the
last currently available 8.2 m blank for a space telescope. This paper will review the outstanding quality of the first four
mirror blanks and present the quality of the blank still available. Additionally we will give an overview over the progress
in the last decade in technology and knowledge and how they might support the use of the 8 m blank as space telescope
mirror.

Pending critical spaceborne requirements, including coronagraphic detection of exoplanets, require exceptionally
smooth mirror surfaces, aggressive lightweighting, and low-risk cost-effective optical manufacturing methods.
Simultaneous development at Schott for production of aggressively lightweighted (&gt;90%) Zerodur<sup>&#174;</sup> mirror blanks,
and at L-3 Brashear for producing ultra-smooth surfaces on Zerodur<sup>&#174;</sup>, will be described. New L-3 techniques for
large-mirror optical fabrication include Computer Controlled Optical Surfacing (CCOS) pioneered at L-3 Tinsley,
and the world's largest MRF machine in place at L-3 Brashear. We propose that exceptional mirrors for the most
critical spaceborne applications can now be produced with the technologies described.

Modeling of the thermal expansion behavior of ZERODUR&reg; for the site conditions of the upcoming Extremely Large
Telescope's (ELT's) allows an optimized material selection to yield the best performing ZERODUR&reg; for the mirror
substrates.
The thermal expansion of glass ceramics is a function of temperature and a function of time, due to the structural
relaxation behavior of the materials. The application temperature range of the upcoming ELT projects varies depending
on the possible construction site between -13&deg;C and +27&deg;C. Typical temperature change rates during the night are in the
range between 0.1&deg;C/h and 0.3&deg;C/h. Such temperature change rates are much smaller than the typical economic
laboratory measurement rate, therefore the material behavior under these conditions can not be measured directly.
SCHOTT developed a model approach to describe the structural relaxation behavior of ZERODUR&reg;. With this model it
is possible to precisely predict the thermal expansion behavior of the individual ZERODUR&reg; material batches at any
application temperature profile T(t). This paper presents results of the modeling and shows ZERODUR&reg; material
behavior at typical temperature profiles of different applications.

Recently SCHOTT has shown in a series of investigations the suitability of the zero expansion glass ceramic material
ZERODUR&#174; for applications like mirrors and support structures of complicated design used at high mechanical loads.
Examples are vibrations during rocket launches, bonded elements to support single mirrors or mirrors of a large array, or
controlled deformations for optical image correction, i.e. adaptive mirrors.
Additional measurements have been performed on the behavior of ZERODUR&#174; with respect to the etching process,
which is capable of increasing strength significantly. It has been determined, which minimum layer thickness has to be
removed in order to achieve the strength increase reliably.
New data for the strength of the material variant ZERODUR K20&#174; prepared with a diamond grain tool D151 are
available and compared with the data of ZERODUR&#174; specimens prepared in the same way. Data for the stress corrosion
coefficient n of ZERODUR&#174; for dry and normal humid environment have been measured already in the 1980s. It has
been remeasured with the alternative double cleavage drilled compression (DCDC) method.

This review paper summarizes the extensive investigations that have been performed at SCHOTT to achieve a deeper
understanding of the CTE homogeneity of ZERODUR® within single blanks and the casted formats (boules). Especially
for the upcoming Extremely Large Telescope (ELT) projects like E-ELT or TMT with at least several hundreds of
mirror segments the reproducibility of the mean CTE, CTE homogeneity and axial gradient is very important while
keeping the CTE quality assurance process economic at the same time. Statistics of CTE homogeneity measurements on
a ZERODUR® boule suitable for an economical production of ELT mirror substrates using the improved dilatometer
will be presented. It will be shown, that it is possible to achieve tight CTE specifications by utilisation of processes
existing at SCHOTT, while at the same time guaranteeing a long term reproducibility. The CTE measurement is
optimized for a temperature interval from 0&deg;C to 50&deg;C. We developed a model to extrapolate the CTE behaviour to
specific temperature conditions at the telescope site.

One of the most important properties of optical glass is the excellent spatial homogeneity of the refractive index of the
material. Nevertheless, sometimes spatially short-range inhomogeneities are formed during the production process.
These striae are strongly anisotropic due to the process of glass melting. In optical systems, they cause degradation of the
performance with a complicated behavior. The quality specification of the glass homogeneity usually is given by simple
values of phase differences along the main propagation direction of the light in an area of a certain size. For the
measurement of these effects, interferometry can be used, which is a quite expensive method in reality. The observation
of striae shadowgraph pictures is a faster and more frequently used method. The evaluation and quantitative
reconstruction of the inhomogeneities in glass based on the striae technique are the main goal of this work. We revise the
experimental setup and develop models to simulate the measurements for thin and thick samples. The results of the
shadowgraph method are compared with interferometric measurements. A more refined evaluation which is not only
based on the image contrast allows a unique and accurate reconstruction of the size and the phase height of striae with
negligible axial extension. A simple procedure shows how one can estimate the effect in thick samples in practice
approximately.

Initiated in 1968 by the first order of Max-Planck-Institute in Heidelberg successful history of ZERODUR<sup>(R)</sup> continues
now since 40 years. ZERODUR<sup>(R)</sup> zero expansion glass ceramic from SCHOTT has been the material of choice in
astronomy for decades, thanks to its special properties such as its extremely high thermal and mechanical stability. Today
most of the major modern optical telescopes of the 4 m class and of the 8 m to 10 m class are equipped with ZERODUR<sup>(R)</sup> .
For the future several Extremely Large Telescope (ELT) projects are in development, which are designed with even
larger primary mirrors ranging from 30 m to 42 m. Also here ZERODUR<sup>(R)</sup> is under consideration. A historical review,
prominent examples of astronomical projects with glass ceramic mirror substrates, and an outlook to the future is given
in this paper.

Drawings of optical elements contain specification requirements on the properties of optical glass in most cases
according to the standard ISO 10110 part 2 stress birefringence, part 3 bubbles and inclusions and part 4 inhomogeneity
and striae. When ordering glass for the production of the elements sometimes uncertainties and misunderstandings
occur, since the specification requirements on elements according to ISO 10110 cannot be transferred to the raw glass
easily and clearly in any case. The reason is that raw glass may be delivered in very different shape: From close to net
shape as pressings to far away from that as strip or block glass. Additionally in many cases an individual inspection by
the glass supplier is not possible due to cost reasons or since it is not known at the time of delivery, which elements will
be produced from the glass piece.
Up to now an international standard was missing for optical raw glass. This standard ISO DIS 12123 is now close to final
voting and release. This presentation provides the motivation for writing the standard, a comparison with ISO 10110 and
information on the state and progress of standardization of optical glass.

Initiated in 1968 by the first order of the Max-Planck-Institute in Heidelberg the successful history of ZERODUR<sup>&reg;</sup>
continues now since 40 years. ZERODUR<sup>&reg;</sup> zero expansion glass ceramic from SCHOTT has been the material of choice
in astronomy for decades, thanks to its special properties such as its extremely high thermal and mechanical stability.
Today most of the major modern optical telescopes of the 4 m class and of the 8 m to 10 m class are equipped with
ZERODUR<sup>&reg;</sup>. For the future several Extremely Large Telescope (ELT) projects are in development, which are designed
with even larger primary mirrors ranging from 30 m to 42 m. Also here ZERODUR<sup>&reg;</sup> is under consideration. A historical
review, the actual status of developments and an outlook to the future is given in this paper.

The new generation of survey telescopes and future giant observatories such as E-ELT or TMT do not only require very
fast or very large mirrors, but also high sophisticated instruments with the need of large optical materials in outstanding
quality.
The huge variety of modern optical materials from SCHOTT covers almost all areas of specification needs of optical
designers. Even if many interesting optical materials are restricted in size and/or quality, there is a variety of optical
materials that can be produced in large sizes, with excellent optical homogeneity, and a low level of stress birefringence.
Some actual examples are high homogeneous N-BK7 blanks with a diameter of up to 1000 mm, CaF2 blanks as large as
300 mm which are useable for IR applications, Fused Silica (LITHOSIL<sup>&reg;</sup>) with dimensions up to 700 mm which are
used for visible applications, and other optical glasses like FK5, LLF1 and F2 in large formats.
In this presentation the latest inspection results of large optical materials will be presented, showing the advances in
production and measurement technology.

The zero expansion glass ceramic material, ZERODUR&reg;, is well known for night-time telescope mirror substrates. Also
for solar telescopes ZERODUR&reg; is often selected as mirror blank material. Examples are the Swedish 1 m Solar
Telescope (SST), the balloon-born telescope SUNRISE, and the New Solar Telescope (NST) of the Big Bear Solar
Observatory. The properties of ZERODUR&reg; are discussed with respect to the special technical requirements of solar
observatories, resulting in the conclusion that mirrors made of this glass ceramic material are an excellent choice for
solar telescopes.

There is a broad range of applications for lightweighted components made from ZERODUR<sup>(R)</sup> glass ceramic. The main
markets are secondary and tertiary mirrors for astronomical telescopes, mirrors and structural components for satellites,
and mechanical structures for industrial applications, mainly in microlithography. Prominent examples from astronomy
are VLT-M3, GEMINI-M2, SOFIA-M1, MAGELLAN-M2, MMT-M2, and METEOSAT-SEVIRI. At SCHOTT
components with blind or undercut semiclosed holes are manufactured, typically with circular, hexagonal, rectangular or
triangular shapes. The classical grinding process results in weight reduction factors of about 70 %. By additional acid
etching technologies even higher lightweighting factors and rib thicknesses below 1 mm have been achieved.

The ZERODUR production, consisting of established processes used in the manufacturing of high homogeneous
optical glasses, results in excellent blanks with low stress birefringence, striae content and outstanding homogeneity of
the coefficient of thermal expansion. For future extremely large telescope projects like OWL (OverWhelmingly Large
Telescope) or TMT (Thirty Meter Telescope), with at least several hundreds of mirror blanks, the material homogeneity
within a single blank is extremely important.
Previously, the stress birefringence of 2m class mirror blanks could be reduced to amounts far below our catalog values.
Striae in ZERODUR, if present, are weak band-like density fluctuations within the material with only minimum
influence on the properties of the material.
This paper presents the results of dilatometric measurements on the influence of standard grade striae within
ZERODUR on the homogeneity of the CTE. All CTE measurements have been carried out using our new dilatometer
setup with improved reproducibility.

ELTs will need large optical lenses for imaging optics and atmospheric dispersion correctors. The extreme dimensions of
lenses considered in designs at present (up to 1.7 m diameter) pose severe challenges for the specification, production and
inspection of the glass blanks. Possible maximum sizes, their very long production time, technical and economical
conditions and probable restrictions are discussed. The inspection of glass blanks for ELT transmission optics can rely on
methods already introduced for optical glass blanks of conventional sizes and for large mirror blanks of the glass-ceramic
ZERODUR(r) for most of their properties. However, especially for the refractive index homogeneity a scale-up still has to
be achieved. At present largest homogeneity interferometers have apertures about 500 to 600 mm. The development of
larger ones is very time consuming, about 2 - 3 years. There is a need for a close agreement on the required capability of
the measurement method between the optical designers and the supplier and to start soon with this since the optical lenses
may turn out to be more critical in production than the segments for the primary mirrors.

For future extremely large telescope projects like OWL or TMT with at least several hundreds of mirror blanks the homogeneity of the coefficient of linear thermal expansion (CTE) within a single blank is an important issue. The telescope designers are not only interested in the global CTE homogeneity but also in measuring the axial CTE gradient to the highest precision.
It has been proven in the past in many projects like GTC and Keck that ZERODUR(r) itself is a material of highest homogeneity even in large dimensions and huge quantities. About 95.5% of all 2m class mirror segments of all projects exhibit a peak to valley homogeneity of better than 0.015*10<sup>-6</sup>K<sup>-1</sup>. The actual homogeneity of the material is even better because the results so far are largely influenced by the restrictions of the CTE measurement repeatability in the past.
This paper introduces an advanced method for the measurement of the CTE of ZERODUR(r) exhibiting a significantly improved reproducibility. The dilatometer setup was especially optimized to cope with the demand of highly accurate homogeneity measurements of 2 m class ZERODUR(r) segments for giant astronomical telescopes.
Detailed measurement results out of a single 1.5 m class ZERODUR(r) segment based on the current state of production will be shown. The results show CTE distributions in radial, angular and axial direction. SCHOTT has already improved the production capacity for ZERODUR(r) immensely, thereby the results represent the current status of quality of the available mass production facilities at SCHOTT.

SCHOTT has a history of more than 35 years with the production of the zero expansion glass ceramic material ZERODUR. More than 250 ZERODUR mirror blanks were already delivered to the large segmented telescopes KECK I, KECK II, HET, GTC, and LAMOST. The increasing worldwide demand on large ZERODUR components for LCD display lithography machines is similar to the expected demand for an Extremely Large Telescope. Last year SCHOTT has ramped up its ZERODUR production capacity. These recent investments in additional melting and ceramisation capabilities are accompanied by improvements of quality assurance and processing technology. SCHOTT is now prepared also for a future production of mirror blanks for Extremely Large Telescopes. The present status of the production capacity and the mass production of ZERODUR mirror blanks for industrial applications are discussed.

The low thermal expansion glass ceramic ZERODUR is the material of choice for many big astronomical telescope projects like VLT, Keck I + II, HET, LAMOST and GRANTECAN (GTC). For future giant telescope projects like OWL or TMT with at least several hundreds of mirror blanks the CTE homogeneity within a single blank and from blank to blank is an crucial issue.
The ZERODUR production process is based on established and proven methods used in the production of high homogeneity optical glasses. Therefore ZERODUR itself is a material of highest homogeneity even in large dimensions and huge quantities. This paper presents an evaluation of the homogeneity of the thermal expansion coefficient within more than 250 mirror blanks. The observed homogeneity range is only slightly larger than the repeatability of the standard dilatometer measurement of ±0.005*10<sup>-6</sup> K<sup>-1</sup>.
To improve the accuracy of measurement and to get a deeper understanding of the thermal expansion behaviour of ZERODUR a new dilatometer was built exhibiting a repeatability of ±0.001*10<sup>-6</sup> K<sup>-1</sup>. Detailed evaluations of the thermal expansion coefficient homogeneity of a 100 mm x 100 mm ZERODUR test block showed no variation within the repeatability of measurement of the improved dilatometer.

VISTA (Visible and Infrared Survey Telescope for Astronomy) is designed to be the world's largest wide field telescope. After finishing of the construction the telescope will be part of ESO and located in Chile close to the VLT observatory at Cerro Paranal. In November 2001 SCHOTT was selected by the VISTA project office at the Royal Observatory of Edinburgh to deliver the 4.1 m diameter primary mirror blank. The manufacturing of the mirror blank made from the zero expansion material Zerodur was challenging especially due to the f/1 design. Several tons of the glass ceramic material were removed during the grinding operation. A meniscus blank with a diameter of 4100 mm and a thickness of 171.5 mm was generated, having a large central hole of 1200 mm and an aspherical shape of the concave surface. Also the handling and turning operations needed special effort and were performed by a skilled team. This paper presents details and pictures of the corresponding production and inspection sequence at SCHOTT. The geometrical parameters were measured during manufacturing by help of a laser tracker system and the achieved parameters were compared with the initial technical specification. The final quality inspection verified the excellent quality of the mirror blank. The close co-operation between the astronomers and industry resulted in a project management without problems. In April 2003 the VISTA blank was delivered successfully within a ceremony dedicated to the anniversary of "100 years of astronomical mirror blanks from SCHOTT."

At present extremely large telescopes are planned with primary mirrors from 20 m up to 100 m. Such telescopes need not only huge mirror arrays but also downstream refractive optics like atmospheric dispersion correctors, color correctors for imaging and beam shapers for spectrometers. For classical boro-crown and lead-flint glass types blanks have been made in the past up to about 1 m. Now there is an increasing demand to be expected for optical glass blanks with diameters up to 1.5 m for use as lenses or prisms. Additionally optics designers ask for glass types like the low dispersion fluoro-phosphate glasses. The production of high quality blanks of such glasses has been a challenge even for diameters around 200 mm. This presentation shall give information about the feasibility of large glass blanks and recommendations how to specify the quality balancing the requirements of the application on one side and the possibilities and conditions of the production and the measurement for inspection on the other side.

SCHOTT has a history of 100 years in delivering mirror blanks for astronomy. Since more than 30 years the zero expansion glass ceramic material ZERODUR is well recognized in the astronomical community. More than 250 ZERODUR mirror blanks for large segmented telescopes have been successfully produced at SCHOTT and were already delivered to KECK I, KECK II, HET, GTC, and LAMOST. For the increasing world wide demand on large ZERODUR components for industrial applications SCHOTT is presently ramping up its production capacity. The investment in additional melting and ceramisation capabilities are accompanied by improvements of quality assurance and processing technology. SCHOTT is now prepared for a future production of ZERODUR mirror blanks for next generation of Extremely Large Telescopes with diameters of 30 m to 50 m. For other large optical elements needed SCHOTT can supply the requested materials like optical glasses, filter glasses, fused silica and calcium fluoride.

For the next generation of X-ray observatories (CONSTELLATION-X and XEUS) a mass production of glass mirror segments is considered. The mirror substrates (SCHOTT D263 and SCHOTT BOROFLOAT 33) will be pre-shaped in a high temperature slumping process by use of precision forming mandrels. SCHOTT GLAS developed the glass ceramic material ZERODUR K20 to meet the requirements of these mandrels. The new material is a modification of the well-known ZERODUR. A heat driven transformation thereby changes the crystalline phase from high-quartz to keatite structure. The resulting ZERODUR K20 exhibits an increased stability at high temperatures of up to 850°C and a low thermal expansion coefficient (CTE) of approximately 20&bull;10<sup>-7</sup> K<sup>-1</sup> (20°-700°C). Numerical simulations of the slumping process based on experimental parameters of Zerodur K20 and the mirror substrate materials are presented.

The XEUS mission (X-ray Evolving-Universe Spectroscopy Mission) is a future ESA project currently under study. With a mirror collecting area of up to 30 m<sup>2</sup> @ 1 keV and 3 m<sup>2</sup> @ 8 keV it will outperform the x-ray space observatories like XMM-Newton. In fact it will have a source flux sensitivity and angular resolution respectively 250 times and 7.5 times better if compared to that mission. This huge collecting area is obtained with a 10 m diameter telescope of 50 m focal length. It is foreseen that the whole telescope will be formed by two free flying satellites, one for the mirror assembly and the other for the detectors. The two satellites will be kept aligned by an active tracking/orbit control system. The angular resolution of the optics is set to 5 arcsec with a goal of 2 arcsec. Of course the requirement of high resolution and large diameter of the optics create new technological problems which have to be overcome. First of all the impossibility to create closed Wolter I shells (due to the large diameter) means that the optics will be assembled using rectangular segments of ~1 m x ~0.5 m size. A set of these segments will form a petal. The petals will be assembled to form the whole mirror assembly. Another difficulty arises from the fact that the current design foresees a mass/geometric-area ratio of 0.08 kg/cm<sup>2</sup>, which is very small and much lower compared with XMM-Newton. Hence the use of materials that can offer both low weight and high stiffness is mandatory. The impossibility to have a thermal control for the huge area of the optics means also that the mirrors have to operate at temperatures between -30 and -40°C. This requirement excludes the epoxy-replication method as option for their manufacturing (CTE mismatch between resin and substrate). Considering all these constrains a possible solution for the realization of the XEUS mirrors has been found that foresees the use of glass or ceramics materials. In this paper we will describe an investigation currently on-going aimed at the development of a procedure to produce large mirror segments from thin Borofloat glass and the preliminary results obtained, that corroborate the viability of the proposed approach. A previous article has introduced the basic ideas and concepts behind this investigation.

Following the actual X-ray satellites XMM-NEWTON and CHANDRA future missions are in discussion. ESA is planning the XEUS-satellite and NASA the CONSTELLATION-X mission. The increasing effective areas of the telescopes require nested thin-walled mirrors of large diameters. For the mass production of segmented shells the techniques of nickel electroforming and of epoxy replication are in evaluation. In both cases ZERODUR glass ceramic was chosen for the replication mandrels due to its high thermal stability and its proven ability to be polished to excellent surface qualities. SCHOTT GLAS has produced pre-shaped prototypes of a-spherical replication mandrels. The final polishing is done at CARL ZEISS, who is also the prime contractor for the finished mandrels. A demonstration mandrel for XEUS has been finished in 2000; the first prototype mandrel for CONSTELLATION-X will be delivered this year. It has been demonstrated that high precision mandrels can be produced with the required accuracy. Thereby ZERODUR is developing from a mirror substrate material (ROSAT, CHANDRA) to the preferred material of mandrels for the replication of X-ray mirrors. This demonstrates the broad variety of applications for this zero expansion glass ceramics.

SCHOTT Glas manufactured more than 200 ZERODUR segments for various Telescopes:
84 pcs. 1.900 mm diameter × 76 mm menisci for Keck I and Keck II,
96 pcs. 1.180 mm hexagonal × 56 mm blanks for HET,
42 pcs. 1.870 mm hexagonal × 83 mm blanks for GTC,
and produces at present,
40 pcs. 1.100 mm hexagonal × 82 mm blanks for LAMOST.
During the production period for the GTC mirror blanks SCHOTT developed improved casting techniques to generate castings with hexagonal shape to realize near net shape processing. This reduces the necessary machining time in a very cost effective manner. Additionally slicing the castings into single blanks reduces the subsequent ceramising time and cost. Results of the important quality characteristics like CTE, CTE homogeneity, internal stress and geometrical dimensions of the GTC blanks will be presented.
Finally the upscaling of the present advanced production technique to the needs of the Extremely Large Telescopes will be discussed.

Schott has delivered blanks for large lenses and prisms since many decades. Glass and glass ceramics objects with dimensions above 300 mm diameter or edge lengths will remain challenges for a glass manufacturer. This holds especially when the quality specifications exceed the standard level significantly. Optical glass blocks of more than half a ton have been produced with outstanding internal quality. Although the manufacturing process is well controlled there are restrictions on the availability of such objects (glass types, long process times e.g.). Implications of the glass production process are presented as a guideline for designers in order to avoid unnecessary time losses. The similarity of the production process of the glass ceramic ZERODUR to that of optical glasses results in high homogeneity with regard to the coefficient of thermal expansion as well as to the optical properties. This qualifies ZERODUR for even higher demanding applications especially when reproducibility in series production is required.

Schott had delivered blanks for large lenses and prisms since many decades. Glass and glass ceramics objects with dimensions above 300 mm diameter or edge lengths will remain challenges for a glass manufacturer. This holds especially when the quality specifications exceed the standard level significantly. The developments in glass manufacturing allow casting of ZERODUR<SUP>R</SUP> glass ceramic blanks up to about 1.5 m with homogeneities like that of high grade optical glass. This was utilized to test a convex mirror using a large Zerodur element in transmission providing the concave interferometer reference surface and the imaging aspherical lens simultaneously. Optical glass blocks of more than half a ton have been produced with outstanding internal quality. Although the manufacturing process is well controlled there are restrictions on the availability of such objects (glass types, long process times e.g.). The glass production process is presented pointing out its implications as a guideline for designers in order to avoid unneccessary time losses.

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